1. Field of the Invention
The present invention relates to methods for the production of polygranular graphite bodies, as well as to the polygranular graphite bodies produced therewith.
2. Description of the Related Art
Polygranular graphite bodies are for example graphite electrodes and connecting pieces (nipples) for graphite electrodes. The following explanations relate in particular to graphite electrodes and nipples, but also apply correspondingly to polygranular graphite bodies other than graphite electrodes or nipples.
Graphite electrodes are used for steel production in an electric arc furnace. Graphite electrodes generally must withstand diverse mechanical stresses. In addition to temperature induced thermal material stresses, these include the mechanical stresses which arise on insulating elements during tilting of the furnace, due to vibration, due to dislocation of scrap during melting, as well as during placement of the strand into the scrap. To withstand the conditions in an electric arc furnace it is therefore generally essential that the relevant graphite electrode has a low coefficient of thermal expansion, low electrical resistance, sufficient density and the properties associated therewith (strength and E-modulus).
Needle cokes, for example petroleum-based or pitch-based needle cokes are generally used in the production for example, of graphite electrodes. The quality and value of needle coke which has a needle-like crystalline structure is determined by many factors such as, for example sulfur and nitrogen content, hardness, ash content, electrical resistance and coefficient of thermal expansion. To produce polygranular graphite bodies, such as graphite electrodes and nipples, two graphitization methods are generally used.
One method—the Acheson-graphitization method—was initially described in U.S. Pat. No. 702,758. The furnace consists of a horizontal bed of refractory materials, furnace heads that carry the current load to the furnace and long side walls, consisting of concrete blocks, steel plates or steel grids. The bottom is cooled by air and loaded with a layer of insulating material, for example granular silicon carbide, metallurgical coke, sand or sawdust. The same materials are used to insulate the side walls and the surface of the furnace.
Due to cheaper production costs however, the second method—the Castner—or lengthwise graphitization method—is the currently commonly used method in the production of graphite electrodes. The material to be graphitized is heated as an ohmic resistance in the direct passage of current to above 2200° C. to 3000° C. The original device for implementation of lengthwise graphitization is the subject of U.S. Pat. No. 1,029,121.
Nipples are used to connect graphite electrodes with one another. The graphite electrodes are herein equipped on their face with threaded boxes into which the nipples are screwed. Production of these nipples occurs generally also on the basis of needle cokes.
One problem that occurs in the use of pitch-based or petroleum-based needle cokes is so-called puffing. Puffing is understood to be the irreversible rapid volume expansion during graphitization in a temperature range of 1500 to 2000° C. that is caused by nitrogen- and sulfur release. This puffing can lead to mechanical stresses in polygranular graphite bodies, resulting in micro- and macro-tears. There is a danger that—due to the formation of tears—polygranular graphite bodies are produced that can only be viewed as rejects.
Because of the irreversible thermal volume expansion of the needle cokes, the process curves for graphitizing of, for example electrodes or nipples must generally be adapted very precisely to the parameters of the needle cokes which in the case of strong puffing behavior leads to longer process times and thus higher production costs. In addition, there may be a greater risk of producing reject products.
Petrol- and pitch-based needle cokes display different levels of puffing behavior. Compared to petroleum-based needle cokes, pitch-based needle cokes have a stronger nitrogen puffing-behavior, since they have a higher nitrogen content compared to petroleum-based needle cokes. During graphitization, this nitrogen is separated from its chemical compound, and largely escapes in gaseous form as elementary nitrogen from the material.
To diminish this puffing behavior, especially with pitch-based needle cokes, inhibitors such as iron oxide are typically added. The effect of iron oxide as an inhibitor is described for example in E. Fitzer et al., “The irreversible expansion behavior of sulfurous cokes in a temperature range of higher then 1000° C.”, High Temperatures—High Pressures, volume 9, pages 243-250, 1977. However, the addition of iron oxide only reduces sulfur puffing, in other words, the addition of iron oxide has no effect upon nitrogen puffing in the graphitization process since the influence of iron oxide relates to the reduction of sulfur puffing
What is needed in the art is a method for the production of polygranular graphite bodies, wherein puffing, especially nitrogen puffing in a temperature range of 1500° C. to 2000° C. is reduced, or ideally completely prevented.